Carbon dioxide (CO2) emissions are a major contributor to the phenomenon of global warming. CO2 is a by-product of combustion and it creates operational, economic, and environmental problems. CO2 emissions may be controlled by capturing CO2 gas before emitted into the atmosphere. There are several chemical approaches to control the CO2 emissions (A. Kohl and R. Nielsen, Gas Purification, 5th ed., Gulf Professional Publishing, Houston, Tex., 1997). However, many of these approaches have drawbacks such as high energy consumption, slow processes, and use of ecologically questionable or toxic compounds.
An enzyme based approach using the capability of carbonic anhydrase to catalyse the conversion of CO2 to bicarbonate at a very high rate (turnover is up to 105 molecules of CO2 per second), overcomes the reaction rates and environmental issues in relation to CO2 capture. Technical solutions for extracting CO2 from gases, such as combustion gases or respiration gases, using carbonic anhydrases have been described in WO 2006/089423, U.S. Pat. No. 6,524,842, WO 2004/007058, WO 2004/028667, US 2004/0029257, U.S. Pat. No. 7,132,090, WO 2005/114417, U.S. Pat. No. 6,143,556, WO 2004/104160, US 2005/0214936, WO 2008/095057. Generally, these techniques operate by bringing a soluble or immobilized carbonic anhydrase into contact with CO2 which either may be in a gas phase or a liquid phase. In the presence of water, carbonic anhydrase catalyses the conversion of CO2 into bicarbonate ions which may be further protonated or deprotonated to carbonic acid and/or carbonate ions depending on the pH of the medium. The ions may either be utilized to facilitate growth of algae or microorganisms that utilize bicarbonate/carbonate as a carbon source, to induce a pH change in a surrounding medium or supply buffering capacity, to provide bicarbonate/carbonate as an active agent for subsequent chemical processes, or precipitated as a carbonate salt, or converted back into pure CO2, which can then be used (for example in enhanced oil recovery, for production of urea, for food and beverage processing, or to supply CO2 to greenhouses or cultivation ponds), released (for example from a contained life support environment such as a submarine, spacecraft, or artificial lung), compressed (for example for transportation through pipelines), or stored (such as in geological or deep oceanic formations or saline aquifers).
Mammalian, plant and prokaryotic carbonic anhydrases (alpha- and beta-class CAs) generally function at physiological temperatures (37° C.) or lower temperatures. The temperature of combustion gasses or the liquids into which they are dissolved may, however, easily exceed the temperature optimum for the carbonic anhydrase used to capture the CO2. One of the drawbacks of using enzyme based solutions is that extensive cooling may be needed in CO2 extraction processes prior to contacting the CO2-containing gas/liquid with the carbonic anhydrase, and cooling is an energy consuming process. Consequently, there is a need for more heat-stable carbonic anhydrases when the enzyme is to be used under industrially relevant conditions.